In a new field called Terra‒Astronomy, we study transient phenomena that may have affected the Earth in some way, such as its climate or biosphere, using both natural archives (for example, 14C in trees, 10Be in polar ice and 60Fe in the ocean crust), as well as historic observations recorded in past centuries or millennia.

Historic observations of transient events are valuable to many different fields within modern astrophysics:

Solar Activity — We have been able to study solar activity in great detail with satellites for a few decades, but telescopic sunspot observations span four centuries and aurorae and (naked-eye) sunspots have been recorded for two millennia. Although solar activity can also be studied using radioisotopes (the solar wind is inversely proportional to cosmic ray influx), their levels depend on the Earth’s magnetic field, which in turn can be independently reconstructed using historic aurorae observations.

Earth Rotation — Lunar and solar eclipses, as well as other conjunctions and occultations that have been recorded with good timing precision in history, are used to study secular variations in the Earth’s rotation.

Comets and Meteors — Historic observations of comets and meteor streams facilitate orbit solutions and the study of their origin.

Historic Supernovae — Pre-telescopic observations of galactic supernovae enable a detailed study of their remnants, including neutron stars and runaway stars. Supernovae ages and celestial locations are well-known from historic observations, and light curve and colour evolution information can also be derived — even light echo spectra can be obtained hundreds of years after the initial explosions. Historic supernovae are the most nearby stellar explosions, so deep follow-up observations can reveal possible former donor stars for Type Ia Supernovae and other previous companions that are now runaway stars. (Note: Gaia is advancing this field significantly.)

Historic Novae — Novae may reveal recurring explosions that can lead to a better understanding of the nova mechanism and thermonuclear supernova progenitors.

In all cases, it is essential to understand correctly historic reports that are written in old or ancient languages — and to note how terminology may have changed. For example, in around AD 1600, the Latin ‘cometes’ not only meant ‘comets’ in today’s sense, but also included novae, supernovae and other transients.

Focus Meeting 5 will bring together astronomers from many different fields, including scholars of the history of astronomy as well as those who already use (or plan to use) historic reports to advance their astrophysical field.

RALPH NEUHÄUSER studied at Ruhr University Bochum, Germany, and the University of Sussex, UK, and worked as a post-doc at the Max Planck Institute for Extraterrestrial Physics, Germany, and at the University of Hawaii, US. He has been a professor of astrophysics at the Friedrich Schiller University Jena, Germany, since 2003 and applies historical observations to modern astrophysical problems, such as solar activity and historical supernovae.